FIGS. 1a and 1b show the voltage U and the current I flowing through a motor vehicle headlight discharge lamp as a function of time t.
Under nominal conditions (starting from T.sub.N), such a lamp is powered by alternating current (AC) having a square waveform at a frequency of about 200 Hz to 1 kHz depending on the lamp. Compared with direct current (DC) operation, AC operation makes it possible to increase the lifetime of the lamp considerably.
The magnitude of the current is servo-controlled so that the power delivered to the lamp is maintained at 35 W.+-.1 W. The voltage is imposed by the lamp, and when operating under nominal conditions it varies over the range 65 V to 125 V.
When starting the lamp, a high voltage trigger pulse V.sub.AM is applied between its electrodes (of the order of 12 kV to 25 kV depending on the lamp). This pulse generates the arc of the lamp (time T.sub.ARC). Thereafter, for a few hundreds of microseconds (until T.sub.AM), the power supply of the lamp is controlled in such a manner as to ensure that the current flowing through the lamp does not drop to zero, during which time the voltage across the lamp drops back down to about 30 V to 100 V depending on the previous state of the lamp.
This trigger stage is then followed for a duration of about 10 seconds (s) to 15 s (from T.sub.AM to T.sub.N) by a stage during which power is raised to 90 watts. The current I is then limited to 2.6 A. This stage of bringing up to power is necessary to heat up the electrodes and to evaporate off halides.
FIG. 2 is a diagram of a power supply circuit enabling such operation to take place. The discharge lamp referenced 1 is connected therein in series with a module 2 for generating the high voltage pulse, and it is fed with voltage from a DC/AC converter 3 connected downstream from a DC/DC converter 4 whose input receives the voltage (12 V) from the vehicle battery B.
The DC/AC converter 3 is an H-connected bridge of four switches 6 controlled by control electronics 7. By way of example, the four switches 6 may be MOS type transistors.
By way of example, the DC/DC converter 4 is a "monoflyback" type circuit as shown in FIG. 2. Such a circuit comprises a transformer 8 whose primary winding 8a is connected in series with a controlled switch Q1 to the terminals of the input voltage source (e.g. the vehicle battery B), while its secondary winding 8b is connected in series with a diode 9 to the terminals of the load to be powered. By way of example, the switch Q1 may be an MOS transistor whose grid is voltage-controlled by the voltage module 7.
The secondary winding 8b charges while the transistor Q1 is on. When the transistor Q1 is off, the primary winding 8a delivers the energy stored in the magnetic element to the load.
The output voltage is proportional to the input voltage multiplied by a ratio that depends on t.sub.on /t.sub.off where t.sub.on and t.sub.off are respectively the on time and the off time of the switch Q1.
The DC/DC converter 4 generates all of the voltages other than the trigger pulse voltage. In particular, it must be capable of supplying a high voltage of 500 V for several milliseconds during startup.
The high voltages are of use only during short periods of time, during startup, but they constrain the components of the converter, and in particular its controlled switch, to be very largely dimensioned.
Unfortunately, it is presently desired to reduce the bulk of power supply circuits for discharge lamps considerably so as to enable them to be housed completely within a headlight, whereas until now the converters of the power supply circuits have beer outside headlights.